In the pulp and paper industry, recovery boilers are used to burn spent liquor from the Kraft pulping process. The concentrated black liquor is burnt in the Kraft recovery boiler to regenerate sodium sulfide and sodium carbonate which is, in turn, converted to sodium hydroxide in a recausticizing plant. The produced white liquor, containing sodium sulfide and sodium hydroxide, is used in pulping wood. Organic matter that is dissolved in the pulping process is destroyed during combustion in the recovery boiler and the heat of combustion is recovered as steam. The steam is used to provide the mill's heat and energy requirements and/or to generate electricity for other uses.
The current practice for introducing combustion air into Kraft recovery boilers involves injection of the air at two or more elevations in the furnace cavity. Moving upward from the bottom or floor of the furnace, the air injection ports at succeeding elevations are referred to as primary, secondary, tertiary and quaternary air. All chemical recovery boilers have at least two levels of air injection. Newer boilers tend to have more levels. At each level, air is injected through port openings found on at least two opposing walls of the boiler. The port openings, which form the air jets, are usually rectangular. Dampers are typically provided to control the effective size of a port opening and the air pressure upstream of the port.
A need exists for a method and an apparatus for optimizing the combustion air system in chemical recovery boilers to improve boiler capacity and combustion uniformity. A need also exists for reducing particulate and gaseous emissions that are generated when the boiler is operating inefficiently.
Thus, an object of the present invention is to provide an optimized, combustion air supply system for a power or recovery boiler that can provide these improvements and eliminate or, substantially reduce operating problems. It is a further object of the present invention to provide an optimized combustion air system for older boiler installations, using existing combustion air ports to improve boiler efficiency and avoid costly boiler modifications. In another aspect of this invention, the optimized combustion air supply system would also be applicable to power boilers having two, three or more levels of combustion air, and can thus be applied in new and previously rebuilt recovery boilers. In yet another aspect, the optimized combustion air supply system would be especially applicable in a Kraft recovery boiler.